Metals such as tungsten are deposited in openings and on the surface of a silicon wafer during manufacture of integrated circuits, preferably using chemical vapor deposition (CVD) methods. The wafer to be processed is supported on a heated susceptor which forms one electrode. The gas manifold plate of the chamber forms the other electrode. A gas, such as WF.sub.6, is supplied through a plurality of openings in the gas manifold plate situate generally parallel to and above the susceptor.
Tungsten is generally deposited using WF.sub.6 gas. Since ions formed from WF.sub.6 are highly mobile and reactive, tungsten also deposits on the edge and backside of the wafer. The backside of a wafer is generally used for registration purposes, and therefore it is important that the wafer backside surface be kept smooth and clean. Thus any tungsten that deposits on the edge or backside of the wafer must be removed. In order to protect the edge and backside of the wafer from such deposition, a clamping ring that overlies the peripheral edge of the wafer to be processed is used to press the wafer against the susceptor, and protect the wafer edge and backside from metal deposition. The use of a clamping ring is advantageous in that only the force caused by the weight of the clamping ring is used against the wafer, which causes a minimum amount of stress on the wafer. However, several problems are still being encountered.
There are various alignment problems between the wafer and the susceptor and between the wafer and the clamping ring.
It is difficult to align the clamping ring so that its center is aligned perfectly both with the center of the wafer and with the center of the susceptor, and to align the center of the wafer with the center of the susceptor. When these elements are misaligned however, it is impossible to produce a uniform tungsten film on the wafer due to non-uniformities of heat transfer from the susceptor to the wafer, and to deposit a tungsten film that is centered with respect to the wafer. Thus the distance of the tungsten film from the edge of the wafer can vary enough so that die near the edge of the wafer may be missed entirely by the metal deposition, reducing the yield from the wafer.
Further, if the clamping ring, due to misalignment, becomes tilted with respect to the surface of the wafer, tungsten can deposit on the edge and backside regions of the wafer.
It is even more difficult to entirely prevent deposition of tungsten on the edge and backside of the wafer, even when using a clamping ring in its correct position.
Studley et al, see U.S. Pat. No. 4,932,358, addressed this problem and discloses an elaborate closure between the clamping ring and the susceptor, producing a strong force pressing the wafer to the susceptor. This strong force can cause stress in the wafer, even leading to damage and cracks in the wafer; and it can cause the susceptor to become deflected from its position parallel to the gas manifold plate, which leads to non-uniformities of the deposited film. A wafer seal ring is mounted on a movable assembly that can be pressed against a wafer mounted on a susceptor by spring means, creating a force of from about 10-40 pounds. The wafer seal is aligned only with respect to the susceptor; the wafer is aligned only by means of pin supports on the susceptor. Thus if the wafer is misaligned with respect to the susceptor, it will also be misaligned with respect to the clamping ring. Further, the clamping ring overlaps the wafer edge by about 3-5 mm. Thus the alignment of the susceptor to the wafer to the clamping ring depends on the sum of the tolerances of these parts, and thus the chances of obtaining a perfect alignment are slim.
Thus it would be highly desirable to provide an improved apparatus and method to prevent deposition of tungsten or other metals on the edge and backside of a wafer during CVD processing, and to readily align the centers of the clamping ring, wafer and susceptor to improve uniformity of the metal film deposited on the wafer.